1. Field of the Invention
The present invention relates to a lithographic apparatus and to a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Between the reticle and the substrate is disposed a projection system that images the irradiated portion of the reticle onto the target portion of the substrate. The projection system includes components to direct, shape and/or control the beam of radiation, and these components typically include refractive optics, reflective optics, and/or catadioptric systems, for example.
A consideration in lithography is the size of features of the pattern applied to the substrate. It is desirable to produce apparatus capable of resolving features as small and close together as possible. A number of parameters affect the available resolution of features, and one of these is the wavelength of the radiation used to expose the pattern.
It is expected that the use of EUV lithography will enable the manufacture of feature sizes down to 32 nm using radiation with an EUV (Extreme Ultra Violet) wavelength between 5 and 20 nm, typically 13.5 nm. Radiation at this wavelength is typically strongly absorbed by most materials and conventional refractive optics are generally considered to be unsuitable for use with such radiation. The optics in a projection system for use with EUV lithography should therefore be based on mirrors, which can only operate in a high vacuum (UHV) environment. The projection system is therefore enclosed in a projection optics box (POB) which is kept under vacuum.
Similar considerations apply for lithography using radiation having a wavelength falling outside the EUV band. For example, a projection system for lithography using radiation having a wavelength of 193 nm may also include mirrors instead of, or in addition to, refractive optics. The POB may therefore need to be kept under a vacuum or at least in a controlled environment for non-EUV lithography.
Furthermore, considerations that apply to the projection system will also apply to the illumination system used to supply the beam of radiation to the reticle. As with the projection system, the illumination system includes components to direct, shape and/or control the beam of radiation, and these components typically include refractive optics, reflective optics, and/or catadioptric systems, for example. As with the projection system, the illumination system may need to be kept in a controlled environment or under vacuum.
The projection system and/or illumination system generally also includes elements to set the numerical aperture, commonly referred to as the “NA”) of the projection system (and/or illumination system. In some prior art systems, an aperture adjustable NA-diaphragm or iris diaphragm may be provided in a pupil of the projection system and/or illumination system. Particularly in the case of EUV lithography, for most common projection optics designs the space around the optical components is very restricted, making the use of an adjustable diaphragm impractical. Furthermore, it is difficult to provide an elliptical diaphragm for off-axis reflective systems.
Yet further, an adjustable diaphragm provides substantially circular apertures. However, different shaped apertures can improve imaging performance for specific structures. For example, an elliptical aperture is useful for off-axis reflective systems. It is not possible to produce apertures having such shape using a conventional diaphragm.